This invention is related to dc-dc type power converters, and is especially directed to power converters of the half-bridge type. The invention is more particularly concerned with a novel current mode controller for a half-bridge power converter.
A half bridge converter, in its most basic terms, has dc power input leads, a pair of capacitors in series across the power input lead and a pair of switches also in series across the power input leads. A transformer has its primary coupled between the junction of the switches and the junction of the capacitors. The transformer secondary has a grounded center tap and end terminals connected through rectifier diodes to an output inductor, followed by a smoothing capacitor and a load resistor.
Switching is controlled by a current mode controller. The current through the transformer primary, i.e., the current through the output inductor, is sensed and a current sense signal is supplied to the controller. The controller turns the switches on in turn and turns them off when the peak sensed current reaches a command level. This produces a drive current with a pulse width through that is controlled as a function of current demand.
The half-bridge converter can employ switching devices one half the voltage rating of other types of converters, and the half-bridge topologies makes more efficient use of the transformer core and windings than other topologies. Leakage inductance energy is returned to the input capacitors, rather than being resistively dissipated. Current-mode control facilitates compensation and gives enhanced large signal performance. Because current-mode control has input voltage feed forward characteristics, this type of control provides good output regulation in the event of input line voltage fluctuation.
Current-mode control has the advantages of reduced output phase delay at the frequencies employed, and typically up to 7000 Hz, as compared with other control systems, such as constant frequency PWM. This gives the converter a rapid response to demand and permits it to operate at peak efficiency.
Half-bridge topology with current-mode control has been considered as an unstable combination because of the difficulty in balancing the capacitors. That is, because the two capacitors will not have exactly the same values, more charge is transferred to one than to the other in each cycle. Current mode control is designed to keep peak current equal in both phases. Consequently, the half cycle with the smaller value capacitor will produce a shorter pulse width. This means that the voltage at the junction of the capacitors will increase or decrease a small amount with each cycle, and after a short time will equal zero or full voltage.
In order to avoid this, the present state of the art employs an auxiliary transformer winding with the same number of turns as the transformer primary winding, but of small diameter wire, and a pair of small high-voltage diodes. The diodes provide a current path to the capacitors to keep them in balance.
While the auxiliary winding and diode arrangement does reduce the instability of the current mode control design, it also reduces the overall efficiency, by as much as 30%. That is an additional 30% of power is consumed in the auxiliary windings and diodes.